So here's my ideas so far for the timeline of JumpVerse. Spoilers btw if you're holding out for that.
Before Dream wakes, Blue and Ink basically fall in love. However, because they're both pretty young and naive about romantic attraction, they both think they're just best friends.
This attraction begins to break when they finally meet Dream, both of them being more attracted to Dream than each other.
Once again, without the three of them really knowing much around romance, they unintentionally form a platonic polycule, not realizing they are closer than most friends would be.
However, during the X Event, Ink becomes friends with Cross, and Cross is the one who tells Ink about romantic relationships. (Mainly because Cross has a crush on Ink but doesn't tell him.)
After the X Event, Blue is upset with Ink for saying he didn't consider him a friend- but his feelings get an even deeper cut when Dream becomes Ink's boyfriend despite Ink's betrayal.
It's only from Ink and Dream officially becoming a couple that Blue realizes he had a thing for both of them. However, he ends up being too embarrassed and caught in his feelings to ever tell either of them about it.
He wants to stay friends with Dream and is deeply afraid that if he confessed his feelings, that he'd lose Dream's friendship.